Low-power, infrared information transmission system

ABSTRACT

A low-power infrared information transmission system, the transmitter of which is compact, portable and capable of being powered by a low-voltage battery power source for an extended period of time, utilizes a unique infrared light pulse position modulation technique. The system is capable of transmitting information from a DC signal to a relatively high frequency AC signal. The system is utilized, for example, to transmit medical information from medical electrodes affixed to a patient via a portable infrared transmitter to a receiver unit which monitors and analyzes the patient&#39;s condition; to provide a remote control unit such as the controller for a video game or a TV on/off/channel selector, and, to provide a portable communications device such as a portable microphone or telephone set.

BACKGROUND OF THE INVENTION

This invention relates to information transmission systems and moreparticularly to an infrared information transmission system with aportable battery operable transmitter.

Infrared information transmission systems have proved invaluable forshort-range transmission of both analog and digital information. Theinherent advantages of infrared transmission are apparent; transmissionof infrared data does not generate radio frequency interference whichmight interfere with other instrumentation; and, even more important,the infrared receiver is unaffected by radio frequency interferenceproduced by other devices present. One known system which utilizesinfrared transmission of analog information is a music system in whichan infrared transmitter is connected to a high-fi, stereo or other musicproduction amplifier apparatus, and a portable receiver is incorporatedin a headphone set. Music is transmitted via infrared energy anywherewithin the confines of the room in which the transmitter is placed. Thetransmitter of these prior art systems, however, is not portable; themodulated FM plus AM envelope approach utilized, requires a relativelylarge quantity of power to transmit infrared signal, and therefore, as apractical matter, cannot be operated from battery power with anyexpectation of a reasonable battery lifetime.

In particular, such applications as a portable transmitter whichtransmits infrared information from a plurality of sensors or medicalelectrodes attached to a patient to provide information to a remotemonitor, by means other than conventional wires, is not feasible usingthe prior art system; nor are portable hand-held analog-digitaltransmitters utilized for video games or television on/off/channelselection capable of being provided utilizing infrared energytransmission. A portable telephone which both receives and transmitsinformation which is connected to the main telephone lines via radiofrequency signal transmission is either very expensive or is not capableof eliminating unwanted interference; infrared transmission has notpreviously been appropriate for this application. In each of the aboveexamples, the transmitter is required to be powered by a small number ofbatteries making the transmitter not only portable but also containablewithin a small (hand-held) package.

It is therefore an object of the present invention to provide animproved information transmission system.

Another object of the invention is to provide an improved infraredinformation transmission system.

A further object of the invention is to provide an infrared informationtransmission system with a portable transmitter.

Still another object of the invention is to provide an infraredinformation transmission system with a portable hand-held transmitterpowerable by a battery power source.

It is a further object of the invention to provide an improved medicalpatient monitoring system.

Yet another object of the invention is to provide an improved wirelessportable microphone or telephone.

It is yet a further object of the invention to provide an improvedhand-held wireless remote control unit for video games, TV set controland the like.

BRIEF DESCRIPTION OF THE INVENTION

These and other objects are accomplished in accordance with the presentinvention in which an infrared information transmission system utilizesa pulse position modulation technique to transmit analog and/or digitalinformation utilizing sufficiently low power to be operable from arelatively small battery power source.

In one embodiment thereof, the transmitter is comprised of an inputdevice coupled to the input of a voltage controlled pulse positionmodulation circuit. The pulse position modulation circuit is coupled atits output to an infrared light-emitting device. A switch preamplifiercircuit is utilized in some embodiments between the input device andmodulation circuit to filter and amplify the input signal from the inputdevice; in some instances, such as the medical application hereindescribed, for example, it is of particular importance that thepreamplifier circuit has very high common mode rejection to filter out60-cycle interference. The pulse position modulator circuit is comprisedof a timer coupled to the signal provided by the input device togenerate output pulses which deviate from a predetermined frequency byan amount corresponding to the voltage level of the input signal. Theinput signal may be a variable DC signal or an AC signal. The output ofthe modulator circuit is coupled to the infrared light-emitting deviceto activate the device in accordance with the generated output pulsesand generate bursts of infrared energy which are in synchronism with theoutput pulses. The receiver is comprised of an infrared photodetectorwhich receives the bursts of infrared energy and a demodulator circuitwhich converts the frequency deviation to an output signal. In apreferred embodiment, the demodulator circuit is comprised of a phaselocked loop phase detector; the photodetector being coupled to the phasedetector by means of an amplifier stage and one or more intermediatefrequency filter stages. At the output of the phase locked loop phasedetector, a voltage is generated which varies in accordance with theinput signal which is generated by the input device. Additional filtersmay be provided at the output of the phase detector depending on theparticular application and environmental conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

Still further objects and advantages of the invention will be apparentfrom the detailed description and claims when read in conjunction withthe accompanying drawings wherein:

FIG. 1 is a perspective view of a video game which incorporates as anintegral part thereof the infrared information transmission system ofthe present invention;

FIG. 2 is a block diagram of the video game of FIG. 1;

FIG. 3a is a graphic representation of the pulses generated over each oftwo distinct channels for the multiple controllers utilized in the videogame of FIG. 1;

FIG. 3b is a graphic representation of the output signal of thedemodulator associated with the first channel;

FIG. 3c is a graphic representation of the output signal of thedemodulator associated with the second channel;

FIG. 4 is a detailed circuit diagram of a universal battery operableinfrared transmitter embodied in the present invention;

FIG. 5 is a detailed circuit diagram of a manual analog input potutilized in the present embodiment to control the video game of FIG. 1;

FIG. 6 is a circuit diagram of an infrared receiver for the firstchannel of the video game of FIG. 1, a second similar receiver beingprovided for the second channel;

FIG. 7 is a perspective view of an EKG monitoring system whichincorporates, as an integral part thereof, the infrared informationtransmission system of the present invention;

FIG. 8 is a block diagram of the EKG monitoring system of FIG. 7;

FIG. 9 is a circuit diagram of a preamplifier/filter circuit utilized tocouple a pair of medical electrodes to the pulse position modulator ofFIG. 4 in the EKG monitoring system of FIG. 7, the pulse positionmodulator circuit being substantially the same as that illustrated inFIG. 4;

FIG. 10 is a block diagram of the receiver section of the EKG monitoringsystem; and

FIG. 11 is a circuit diagram of the input section of the receiverutilized in the EKG monitoring system, the phase detector section beingsimilar to that illustrated in FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring then to the drawings, two embodiments of the infraredtransmission system of the present invention are shown in detail; avideo game system embodying the present invention is illustrated in FIG.1 and a medical condition monitoring system, in particular, an EKGmonitoring system embodying the present invention, is illustrated inFIG. 7. The video game system will first be described with respect toFIGS. 1-6.

Referring then to FIG. 1, a video game system is comprised of a maingame module 10 housing the video game with an infrared receiver sectionfor receiving and transferring an input control signal to the video gamecircuit. The output of the video game circuit is coupled to the antennaconnection of standard television receiver 11, a pair of battery-poweredhand control units 12A and 12B is provided for generating the controlsignals which are transmitted in the form of infrared energy bursts tothe receiver section of main game module 10. Each of the battery-poweredhand controls 12A, 12B includes a manual joystick 13A, 13B which may beof the analog (resistor pot controlled voltage) or digital type and mayinclude a keyboard 14A, 14B for entry of additional informationgenerally in digital form. Thus, in accordance with the presentinvention, the battery-powered hand controls are coupled to the maingame module 10 of the video game solely by means of an infrared linkrather than the standard cables presently employed in video gamesystems. In addition, and unlike other systems employing infraredinformation transmission, hand controls 12A, 12B are portable, operablesolely on battery power which, in the described embodiment, may be asingle 9-volt transistor radio type battery.

A block diagram of the video game system of FIG. 1 is illustrated inFIG. 2. Referring then to FIG. 2, each of the controllers 12A and 12Binclude at least one input means which, in the present embodiment,includes a manual analog pot-joystick controlled input circuit 15A, 15B(although, in other embodiments, digital-type joysticks may be utilized)and optionally, digital-coded input devices such as keyboard 16A, 16B. Apulse position modulator circuit 17A, 17B is provided in each ofcontrollers 12A and 12B with an input thereof being coupled to the inputmeans 15A, 15B to receive a variable voltage input signal generated bythe input device. Pulse position modulator circuit 17A, 17B includes atiming means for generating pulses controllably positioned in timewithin a predetermined modulation range from a reference frequency independence upon the voltage level of the input signal. The referencefrequencies for the controllers 12A and 12B are sufficiently differentto prevent overlapping; in the present embodiment, controller 12A is setto a reference frequency of 5 kHz (channel I) and controller 12B is setat a reference frequency of 7 kHz (channel II) as illustrated in thegraphic representation of FIG. 3a. An infrared light-emitting devicesuch as a light-emitting diode is coupled to pulse position modulationcircuit 17A, 17B for receiving the pulsed output signal, thelight-emitting device 18A, 18B generating infrared energy bursts insynchronism with the modulated output pulses.

The receiver is comprised of at least one infrared photodetector 19which detects the bursts of infrared energy generated by photoemitters18A and 18B and a channel separator 20 which distinguishes between thesignals transmitted from controller 12A and controller 12B. In thepresent embodiment, as will later be discussed in detail with respect toFIG. 6, separate photodetectors may be provided for each channel, aphotodetector for channel I being coupled to a 5-kHz bandpass filter anda second photodetector for channel II being coupled to a 7-kHz bandpassfilter. A pair of demodulator circuits 21A and 21B are provided.Demodulator circuit 21A generates an electrical signal indicative of thedeviation of the pulses of the modulator output signal from the channelI 5-kHz reference frequency, and demodulator circuit 21B generates anelectrical signal indicative of the deviation of the pulses of themodulator output signal from the channel II 7-kHz reference frequency.Thus, demodulator circuit 21A applies a signal indicative of the outputof manual analog input joystick 15A to game circuitry 22, anddemodulator circuit 21B provides an output signal indicative of theposition of manual analog input joystick 15B to video game circuitry 22.The channel I output signal from demodulator circuit 21A, as a functionof input frequency deviation, is graphically illustrated in FIG. 3b, andthe channel II output signal from demodulator circuit 21B, as a functionof input frequency deviation, is graphically illustrated in FIG. 3c.

With reference to FIGS. 4-6, detailed circuit diagrams of the infraredinformation transmission system, utilized in the video game embodyingthe present invention, is shown. The transmitter is designated as auniversal transmitter as it is capable of providing a pulse positionmodulated signal from an input signal which is either a variable DCvoltage, as utilized in the presently described video game embodiment,or amplitude or frequency modulated AC as will be seen with respect to amedical monitoring system later described herein with respect to FIGS.7-11.

Referring then to FIG. 4, the transmitter is comprised of a voltagecontrolled timing circuit 23 such as the SN72555 manufactured and soldas a standard product by Texas Instruments Incorporated, or the LM555timing circuit manufactured and sold by National Semiconductor. Thetransmitter is powered by a battery power source B such as a single9-volt transistor battery (nominal operating voltage of the transmitterbeing 6-9 V DC). The reference frequency of timer 23 is adjusted bymeans of resistors R₄ and R₅ and capacitor C₄ ; thus, for controller12A, timer 23 is adjusted to 5 kHz, and for controller 12B, timer 23 isadjusted to 7 kHz. Variance in frequency with ambient (temperature)conditions and change of supply voltage is prevented by means of avoltage regulator circuit comprised of diode D1, zener diode D2,resistors R1 and R2, capacitor C2 and transistor Q1 which provides astable operating voltage V_(CC) from battery voltage V_(BB) provided bybattery power source B. The output of timer 23 is a train of pulsesselectively modulated from the reference frequency in accordance withthe voltage level of input signal provided at terminal 5 of timer 23.The output of timer 23 is provided to the base of transistor Q2 by meansof capacitor C₅. Transistor Q2 switches transistor Q7 in synchronismwith the output of timer 23. Transistor Q7 is switched, capacitor C₁ isdischarged through transistor Q7, thereby driving infraredlight-emitting diode LED with the charge stored in capacitor C₁.Light-emitting diode LED thereby generates infrared energy bursts insynchronism with the modulated output pulses from timer 23.

Separate transmitter devices, as shown in FIG. 4, are provided for eachof control units 12A and 12B as indicated above. In the presentembodiment, a joystick pot 13A, 13B, a circuit diagram of which isillustrated in FIG. 5, is utilized to provide the controlled variablevoltage input signal to timer 23. The analog joystick circuit iscomprised of a variable resistor R53 coupled between positive supplyvoltage of low internal impedance and ground to provide a DC signal atits output which varies approximately linearly between the positivesupply voltage and ground voltage in dependence upon the position of thejoystick 13A, 13B. The joystick, which is now coupled to a video game bymeans of infrared energy transmission, can, for example, controlvertical movement of an image upon the video screen to simulate a racketwith which a moving image of a ball is volleyed. By adding additionalchannels, horizontal as well as vertical positioning is realized; insuch instance, additional reference frequencies are utilized; 5 kHz and7 kHz might be utilized for vertical positioning of the two respectiverackets, and 9 kHz and 11 kHz utilized for horizontal positioning of thetwo respective rackets. Other channels may be utilized for transmittingdigital signals from a keyboard, for example. This is accomplished byselective deviation of the transmitted pulses from a preselectedreference frenquency; for example, a negative deviation might indicate azero bit and a positive deviation, a one bit. The signals aredemodulated in the receiver to determine the activated keys and the gamecontrolled accordingly.

In the presently described two-channel embodiment, two receivers areutilized. Each receiver generates a DC output voltage which is coupledto a control input of the video game electronics 22; these controlinputs are the same terminals to which the pot/joystick circuitillustrated in FIG. 5, is normally directed connected to conventionalvideo games by means of a cable.

Referring then to FIG. 6, the receiver includes a photodetector circuitcomprises of resistors R25-R28, capacitor C11 and infrared photodetectortransistor Q3 which receives the bursts of infrared energy generated bythe infrared light-emitting diode LED and provides a pulsed electricalsignal via capacitor C12 to a preamplifier circuit. The preamplifiercircuit is comprised of resistors R30-R33 and transistor Q4. The outputof the preamplifier circuit is transferred via capacitor C13 andresistor R34 to a bandpass tuned amplifier filter comprised of resistorsR35-R39, capacitors C14-C17, coils L2 and L3 and operational amplifiersA3 and A4. Bandpass tuned amplifier circuit is tuned to, for example, 5kHz for channel I and 7 kHz for channel II. The filtered signal is thentransferred via capacitor C19 to a phase lock loop phase detector 24which detects the leading edge of the transferred signal. The phase lockloop phase detector may be the LM 565 manufactured and sold as astandard part by National Semiconductor. The adjusted output voltageprovided at terminal 7 of detector 24 and a reference voltage providedat terminal 6 of detector 24 are provided to operational amplifier A5which, in turn, provides the DC output voltage to the video game asdiscussed above. The voltage controlled output signal from phase lockedloop phase detector 24, as previously mentioned, is illustrated in FIG.3B and 3C for channels I and II, respectively.

Another embodiment of the invention is utilized as an integral part of amedical condition monitoring system, in particular, an EKG monitoringsystem in which an AC signal derived from the skin of a heart patient ina hospital, etc, is transmitted via infrared energy to a receiver whichis located at the patient's room. The receiver may be part of, orremotely located from, heart monitoring equipment by which the patient'scondition is determined and monitored. The EKG monitoring system willnext be described in detail with respect to FIGS. 7-11.

Referring then to FIG. 7, an EKG monitoring system includes a pair ofmedical electrodes 30 and 31 which are silver chloride/silver galvanicsensors which sense voltage produced by the body which are on the orderof 1 mV. In accordance with the present invention, the medicalelectrodes are connected to a portable battery-powered infraredtransmitter 32 which amplifies and transmits the electrical signalsdetected by electrodes 30 and 31 to EKG monitor 32A. As shown in theblock diagram of FIG. 8, transmitter 32 is comprised of pulse positionmodulator circuit 25 coupled at its input to the medical electrodes andat its output to infrared light-emitting device 26. EKG monitor 32Aincludes an infrared photodetector which receives the informationtransmitted by infrared light-emitting device 26. Photodetector 27converts the infrared energy into an electrical signal which isdemodulated by demodulator circuit 28. The output signal of demodulator28, which is a representation of the signal provided to the system bymedical electrodes 30 and 31, is displayed on a hard or soft copydisplay 29. Display 29 may be remote from photodetector 27; for example,photodetector 27 may be located in a patient's hospital room while thedisplay may be located at a central monitoring location.

Referring to FIG. 9, a detailed circuit diagram of the input stage oftransmitter 32 is shown. Medical electrodes 30 and 31 are connectedbetween terminals 33 and 34, and are coupled to the inputs ofdifferential amplifier A1 by means of capacitors C7 and C8. Amplifier A1is connected by means of capacitors C9 and C10 and resistors R17 and R18to a second operational amplifier A2. Amplifiers A1 and A2 serve toamplify the millivolt signal provided by the medical electrodes, andserves as a filter with high common mode rejection, on the order of 100dB, to substantially filter out 60-Hz interference generated by nearlyelectrical power lines.

The output signal from amplifier A2 is coupled to the input of theuniversal pulse position modulator transmitter illustrated in FIG. 4 bymeans of resistor 22 and capacitor C5. The signal generated by themedical electrodes is on the order of 300 to 400 cycles AC which is thefrequency of the input signal to the pulse position modulator circuit ofFIG. 4. In this application, timer 23 is adjusted to a referencefrequency of 44 kHz. Timer 23 thus generates, at its output, a pulseposition modulated signal, the pulses of which are controllablypositioned in time from the 44-kHz reference frequency signal by adeviation which depends upon the varying amplitude of the input signalprovided by the medical electrodes. The infrared light-emitting deviceLED is coupled to the output of the output of timer 23 by means oftransistors Q2 and Q7, so that the light-emitting device generatesinfrared energy bursts in synchronism with the pulses output by timer23.

The receiver is illustrated in the block diagram of FIG. 10. Referringto FIG. 10, infrared photodetector 35 receives the pulse modulatorinfrared energy bursts generated by the infrared light-emitting diode ofthe transmitter of FIG. 4 and generates a pulsed electrical signal insynchronism therewith. The output signal from the photodetector isamplified and filtered in an intermediate frequency filter 36, theoutput signal of which is provided to a phase locked loop phase detector37. Phase detector 37 generates an electrical signal which, in thisembodiment, is an AC signal, the amplitude and frequency of which variesin accordance with the input signal provided by medical electrodes 30and 31. The output of phase detector 37 is passed through a 60-Hz notchfilter 38 which filters out interference which may be caused by powerlines in the vacinity of the receiver and transmitter. Since thefrequency of the signal generated by the medical electrodes is in therange of 300 to 400 cycles which is essentially the same frequencysignal provided at the output of phase detector 37, a low pass filter 39is further coupled between the phase locked loop detector and the EKGmonitoring equipment.

A detailed diagram of photodetector circuit 35 and intermediatefrequency filter 36 is shown in FIG. 11; phase detector 37 isessentially identical to the phase locked loop phase detector circuitillustrated with respect to FIG. 6 and 60-Hz notch filter 38 and lowpass filter 39 are of conventional design, and will not here be furtherdescribed.

The EKG monitoring equipment, which is comprised of a CRT or hardcopydisplay apparatus, is also of conventional design, and will not bediscussed in further detail here.

Referring then to FIG. 11, photodetector circuit 35 is comprised of aphototransistor Q5 which is prebiased by means of resistors R54, R56 andR58. Transistor Q5 provides a pulsed electrical signal in synchronismwith the infrared energy bursts transmitted by the infrared LED, viacapacitor C24, to the base of amplifying transistor Q6. The amplifiedoutput signal from transistor Q6 is transferred by means of capacitorC25 and resistor C62 to intermediate frequency filter 36 which isessentially comprised of a differential amplifier circuit A5 and anoperational amplifier circuit A6. The output of operational amplifiercircuit A6 is transferred by means of capacitor C29 to a phase lockedloop phase detector 37.

The values of the components, as utilized in each of the above-describedexamples is given below in TABLE I.

Various embodiments of the infrared information transmission system ofthe present invention as well as systems which include various aspectsof such infrared transmission system as an integral part thereof havenow been described in detail. It is contemplated that other systemsincorporating the present invention may include a microphone ortelephone transmitter as the input device which provides an input signalto the universal transmitter shown and described with respect to FIG. 4so that such microphone or telephone may be made wireless. The presentinvention is also adaptable for use in conjunction with any relativelyshort-distance analog or digital information transmission where it isdesirable for the transmitter of such system to be battery operable andportable.

Since it is obvious that many changes and modifications can be made inthe above details without departing from the nature and spirit of theinvention, it is understood that the invention is not to be limited tosaid details except as set forth in the appended claims.

                                      TABLE I                                     __________________________________________________________________________    COMPONENT                                                                             VALUE  COMPONENT                                                                             VALUE  COMPONENT                                                                             VALUE                                   __________________________________________________________________________    Universal Transmitter                                                         R1      1.2 KΩ                                                                         7       100 KΩ                                                                         C2      125 μF                               R2      33 KΩ                                                                          R8      820 ohm                                                R3      33 KΩ                                                                          R9      100 KΩ                                                                         C4      ADJ TO f                                R4      ADJ TO f                                                                             R10     16 KΩ                                                                          C5      2 μFnon                              R5      1.8 KΩ                                                                         R11     2.7 ohm                                                                              C6      2 μF polar                           R6      33 KΩ                                                                          C1      640 μF                                              Medical Electrode                                                             Preamplifier/Filter                                                           R12     220 KΩ                                                                         R19     220 KΩ                                                                         C8      1 μF                                 R13     220 KΩ                                                                         R20     220 KΩ                                                                         C9      1 μF                                 R14     51 KΩ                                                                          R21     47 KΩ                                                                          C10     1 μF                                 R15     51 KΩ                                                                          R22     680 ohm                                                                              C5      10 μF                                R16     12 KΩ                                                                          R23     220 KΩ                                                                         A1      CA-3000 (RCA)                           R17     22 KΩ                                                                          R24     680 KΩ                                                                         A2      SN.sup.2 5741                           R18     22 KΩ                                                                          C7      1 μF                                                Video Game Receiver                                                           R25     150 KΩ          C13     .1 μF                                R26     15 KΩ           C15     .1 μF                                R27     33 KΩ                                                                          R42     2.7 KΩ                                                                         C17     .1 μF                                R28     15 KΩ                                                                          R43     2.7 KΩ                                                                         C18     .003 μF                              R30     330 KΩ                                                                         R44     5 KΩ                                                                           C19     .1 μF                                R31     27 KΩ                                                                          R45     10 KΩ                                                                          C20     .001 μF                              R32     5.6 KΩ                                                                         R46     to requirem't                                                                        C21     to requirem't                           R33     270 ohm                                                                              R47     47 KΩ                                                                          C22     25 μF                                R34     2.2 KΩ                                                                         R48     47 KΩ                                                                          A2      SN72748                                 R35     2.2 KΩ                                                                         R50     390 KΩ                                                                         A4      SN72748                                 R36     39 KΩ                                                                          R51     100 KΩ                                                                         A5      SN72741                                 R37     330 KΩ                                                                         R52     470 KΩ                                                                         R53     5 KΩ                              R38     2.2 KΩ                                                                         C11     1-2 μF                                                                            L1      37 mH.sub.Y                             R39     39 KΩ                                                                          C12     .1 μF                                                                             L2      37 mH.sub.Y                                                           L3      37 mH.sub.Y                             Input Stage                                                                   EKG Receiver                                                                  R54     15 KΩ                                                                          R63     1 KΩ                                                                           C26     8 pF                                    R55     150 KΩ                                                                         R65     68 KΩ                                                                          C27     ADJ to f                                R56     100 KΩ                                                                         R66     1 KΩ                                                                           C28     1000 pF                                 R57     12 KΩ                                                                          R67     39 KΩ                                                                          C29     .05 μF                               R58     150 KΩ                                                                         R68     68 KΩ                                                                          C30,31  10 μF each                           R59     5.6 KΩ                                                                         C22     .1 μF                                                                             Q5      TIL 81                                  R60     330 KΩ                                                                         C23     1 μF                                                                              Q6      2N930                                   R61     33 KΩ                                                                          C24     820 ρF                                                                           A5      SN72748                                 R62     240 Ω                                                                          C25     .05 μF                                                                            A6      SN72740                                 __________________________________________________________________________

This invention relates to copending patent applications, Ser. No.791,913, entitled "Video Game With Portable Control Units" and Ser. No.791,899, entitled "Medical Patient Condition Monitoring System" both bythe inventors of the present invention assigned to the assignee of, andfiled of even date with, the present invention.

What is claimed is:
 1. An infrared information transmission systemcomprising:(a) a low-power portable transmitter module including:(i)data input means for receiving an input signal representative of data tobe transmitted; (ii) battery means for providing power from a batterysource to power said transmitter module; (iii) a pulse positionmodulator circuit coupled to said data input means, said pulse positionmodulator circuit including timing means for generating pulsescontrollably positioned in time, within a predetermined modulation rangefrom a reference frequency in dependence upon said input signal; and(iv) an infrared light-emitting device coupled to said pulse positionmodulation circuit for receiving pulses generated thereby, saidlight-emitting device generating infrared energy bursts in synchronismwith the pulses generated by said modulator circuit; and (b) a receivermodule including:(i) an infrared photodetector device for altering anelectrical signal in accordance with received bursts of infrared energygenerated by said light-emitting device to provide an electrical signalpulsed in synchronism with said bursts; (ii) a demodulator circuitcoupled to said photodetector device for generating an output signalindicative of the deviation of the pulses of said electrical signal fromsaid reference frequency; and (iii) means coupled to said demodulatorcircuit, said means being controlled by said output signal.
 2. Theinfrared information transmission system according to claim 1 includinga voltage regulator coupled to said pulse position modulator circuit forproviding an approximately stable reference voltage to said timing meansregardless of variations in ambient conditions.
 3. The infraredinformation transmission system according to claim 1 including drivercircuit means coupling said pulse position modulator circuit to saidinfrared light-emitting device.
 4. The infrared information transmissionsystem according to claim 3, wherein said driver circuit includescapacitor means for storing a charge during the time periods when nopulse is being transmitted by said modulator circuit and for discharginginto said light-emitting device upon the occurrence of the generatedpulses.
 5. The infrared information transmission system according toclaim 1, wherein said light-emitting device is an infraredlight-emitting diode.
 6. The infrared information transmission systemaccording to claim 1, wherein said infrared photodetector is an infraredphototransistor.
 7. The infrared information transmission systemaccording to claim 1 including amplifier means coupling saidphotodetector to said phase detector circuit.
 8. The infraredinformation transmission system according to claim 1 including anintermediate bandpass filter coupling said photodetector to said phasedetector circuit.
 9. The infrared information transmission systemaccording to claim 1, wherein said phase detector circuit is a phaselocked loop phase detector circuit.
 10. The infrared informationtransmission system according to claim 1 including a notch filtercoupling said phase detector circuit to said controlled means forrejecting AC interference from power lines.
 11. The infrared informationtransmission system according to claim 1, wherein said infraredphotodetector is an infrared photo diode.